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Ocean isotopic evaporation models, such as the Craig‐Gordon model, rely on the description of nonequilibrium fractionation factors that are, in general, poorly constrained. To date, only a few gradient‐diffusion type measurements have been performed in ocean settings to test the validity of the commonly used parametrization of nonequilibrium isotopic fractionation during ocean evaporation. In this work, we present 6 months of water vapor isotopic observations collected from a meteorological tower located in the northwest Atlantic Ocean (Bermuda) with the objective of estimating nonequilibrium fractionation factors (k, ‰) for ocean evaporation and their wind speed dependency. The Keeling Plot method and Craig‐Gordon model combination were sensitive enough to resolve nonequilibrium fractionation factors during evaporation resulting into mean values ofk₁₈ = 5.2 ± 0.6‰ andk₂ = 4.3 ± 3.4‰. Furthermore, we evaluate the relationship betweenkand 10‐m wind speed over the ocean. Such a relationship is expected from current evaporation theory and from laboratory experiments made in the 1970s, but observational evidence is lacking. We show that (a) in the observed wind speed range [0–10 m s⁻¹], the sensitivity ofkto wind speed is small, in the order of −0.2‰ m⁻¹ s fork₁₈, and (b) there is no empirical evidence for the presence of a discontinuity between smooth and rough wind speed regime during isotopic fractionation, as proposed in earlier studies. The water vapord‐excess variability predicted under the closure assumption using thekvalues estimated in this study is in agreement with observations over the Atlantic Ocean.

 

This addendum to the Scientific Prospectus of the International Ocean Discovery Program (IODP) South China Sea Rifted Margin Expeditions 367 and 368 (Sun et al., 2016) addresses a change of port-of-call and a two-day extension to Expedition 368 and modifications to some of the primary and alternate sites for both expeditions. As of the date of publication of this addendum, Expedition 368 is scheduled to start in Hong Kong on 9 April 2017 and end in Shanghai on 11 June. The two-day extension allows for the additional transit to Shanghai at the end of the expedition without taking time away from scientific drilling operations. 

Several recent studies from both Greenland and Antarctica have reported significant changes in the water isotopic composition of near‐surface snow between precipitation events. These changes have been linked to isotopic exchange with atmospheric water vapor and sublimation‐induced fractionation, but the processes are poorly constrained by observations. Understanding and quantifying these processes are crucial to both the interpretation of ice core climate proxies and the formulation of isotope‐enabled general circulation models. Here, we present continuous measurements of the water isotopic composition in surface snow and atmospheric vapor together with near‐surface atmospheric turbulence and snow‐air latent and sensible heat fluxes, obtained at the East Greenland Ice‐Core Project drilling site in summer 2016. For two 4‐day‐long time periods, significant diurnal variations in atmospheric water isotopologues are observed. A model is developed to explore the impact of this variability on the surface snow isotopic composition. Our model suggests that the snow isotopic composition in the upper subcentimeter of the snow exhibits a diurnal variation with amplitudes in δ¹⁸O and δD of ~2.5‰ and ~13‰, respectively. As comparison, such changes correspond to 10–20% of the magnitude of seasonal changes in interior Greenland snow pack isotopes and of the change across a glacial‐interglacial transition. Importantly, our observation and model results suggest, that sublimation‐induced fractionation needs to be included in simulations of exchanges between the vapor and the snow surface on diurnal timescales during summer cloud‐free conditions in northeast Greenland.